1. Field of the Invention
The present invention relates to a fabrication method for a mask ROM device. More particularly, the present invention relates to a low thermal budget fabrication method for a mask read only memory (ROM) device.
2. Background of the Invention
Mask ROM device is the most fundamental type of read only memory device. The conventional fabrication method for a mask ROM device includes forming a buried bit line, followed by forming a gate oxide layer and a gate layer, etc. However, the gate oxide layer is formed at a temperature of about 800 degrees Celsius. Lateral diffusion of dopants in the bit line thus easily occurs due to the thermal process. As the device dimension continues to scale down, lateral diffusion of dopants resulted from the thermal process generated in the forming of the gate oxide layer becomes more serious.
Therefore, a so-called post-gate oxide mask ROM fabrication method is currently being proposed, in which a gate oxide layer is first formed on a substrate. A gate layer is then formed on the gate oxide layer, followed by forming a silicon nitride layer as a hard mask layer. Thereafter, etching is performed to remove a portion of the silicon nitride layer and the gate layer. Ion implantation is further performed to form the buried bit line.
Although the aforementioned “post-gate oxide” fabrication method can prevent the various thermal process induced lateral diffusion problems, this type of fabrication method is much more complicated than the conventional approach, greatly increasing the production cost and the processing time.